As the volume of letter mail becomes increasingly heavy, more attention is being directed to optical reading systems to assist in the handling thereof. For example, an escort code consisting of a plurality of bars and half bars, arranged to provide desired information, may be applied to the faces of the respective documents at different stages of their processing. The code may provide ZIP information needed for interim sorting to route the mail pieces to their destination city or postal station. Also, the code may be used to provide sort information to the carrier, firm, post office box or building--direct level. Optical code readers additionally have significance to large mailers for implementing pre-sorting requirements.
A convenient, economical manner of applying the bar code to the document being processed is through the use of an ink jet printer. In such printing, the particles of ink are sprayed onto the surface to be marked, and generally do not result in the monolithic, precisely formed bars achieved with other printing methods, such as lithography. Optical reading systems exist which operate satisfactorily with bar codes of the last mentioned type, but produce erroneous results when detecting code of relatively poor quality provided by ink jet methods. Obviously, what is optimally required is a cost effective reader system which can accurately recognize bar code generated by both processes.
The recognition circuit of the present invention fills such a need. It enables the optical reader system in which it is used, to reject extraneous ink dots and to ignore minor print voids. It is also skew tolerant and independent of the position and velocity of the bars being detected.